The present invention relates to an image generating method, an image generating apparatus and a radiation tomographic imaging apparatus which fan-para convert radiation projection data and perform back projection thereon to thereby reconstruct an image, and a program therefor.
In a third-generation X-ray CT (Computed Tomography) apparatus, X rays of a fan beam are used in the acquisition of projection data. There is a case in which the so-acquired fan beam projection data are converted to parallel beam projection data, followed by being subjected to a back-projection process to reconstruct an image.
This conversion is generally referred to as “fan-para conversion”. An image reconstructing method using the fan-para conversion is called “fan-para conversion method”. The fan-para conversion method is principally performed with the aim of ensuring the uniformity of CT values, suppressing artifacts at the time that MPR (Multi-Plane Reconstruction) is carried out, reducing a computational processing amount, and so on.
On the other hand, when fan beam projection data of plural views are taken apart every data in a channel direction and rearranged in a simplistic form to acquire parallel beam projection data, the positions of radiation paths corresponding to the respective data are unequally spaced in the channel direction.
Since, however, Fourier transform corresponding to frequency conversion is performed on the parallel beam projection data after the fan-para conversion in the fan-para conversion method, the radiation paths corresponding to the respective data need to be arranged at equal intervals in the channel direction.
Thus, normally, when the fan-para conversion is performed, not only a rearrangement process but also an interpolation process in the channel direction is performed on the fan beam projection data to acquire equally-spaced parallel beam projection data in which the positions of radiation paths are arranged at equal integrals in the channel direction (refer to, for example, paragraph [0004] of Japanese Unexamined Patent Publication No. 2012-005757).
When, however, the equally-spaced parallel beam projection data acquired by performing the interpolation process in the channel direction are compared with pre-interpolation process data (i.e., data before the interpolation processing), more errors from true values are included therein, thereby leading to a reduction in the spatial resolution of a reconstructed image.
With the foregoing in view, there has been a demand for a technology that enables a reduction in the spatial resolution of the reconstructed image to be suppressed even if the fan-para conversion is performed.